(1) Field of the Invention
The present invention relates to a multibeam scanning device which is used in a multicolor image forming apparatus, such as a color copying machine and a laser printer.
(2) Related Art
A tandem-type color copying machine, as one example of a conventional multicolor image forming apparatus, has photosensitive drums corresponding to reproduction colors set along a transporting surface of a transport belt. The reproduction colors are cyan, magenta, yellow, and black, which are respectively referred to as C, M, Y, and K hereinafter, and components related to these colors are assigned numerals with a corresponding C, M, Y, or K. A multibeam scanning device has four laser beams emitted, with each laser beam scanning the surface of the corresponding photosensitive drum. As a result, electrostatic latent images corresponding to reproduction colors are respectively formed on the photosensitive drums. The electrostatic latent images are respectively developed into visible toner images using the reproduction color toner. Then, the toner images are sequentially transferred onto a recording sheet to form a color image, with the recording sheet being transported on the transport belt.
A beam scanning device used in such a color copying machine is composed of four laser diodes corresponding to four reproduction colors and an optical system which deflects laser beams emitted from the laser diodes to expose the corresponding photosensitive drums. The optical system includes optical elements, such as a polygon mirror, a scanning lens, and redirecting mirrors.
Each laser diode is driven in accordance with inputted image data and emits a laser beam. The laser beam is reflected off the surface of the polygon mirror which is rotating and deflects the laser beam. Then, the laser beam exposes the corresponding photosensitive drum via lenses, such as the scanning lens.
Here, if a polygon mirror or a scanning lens are provided for each laser beam, cost increases and the construction of the beam scanning device is increased in size.
A multibeam scanning device using one polygon mirror has been suggested. Japanese Laid-Open Patent Application No. 8-271817 teaches an example of such a multibeam scanning device.
FIG. 1 shows a front view of the schematic construction of the multibeam scanning device disclosed in the cited Japanese application.
A multibeam scanning device 100 shown in FIG. 1 has a polygon mirror 101. Four laser beams LK to LC are respectively emitted from four laser diodes (not shown) to one mirror face of the polygon mirror 101 as shown in FIG. 1. The laser beams LK to LC are linearly aligned in a direction parallel to a rotational axis O of the polygon mirror 101, that is, the laser beams LK to LC are aligned in the sub-scanning direction. The deflected laser beams LK to LC intersect with one another at a position P between a toroidal lens 102 and an f.theta. lens 103, with the position P being located on a plane Q which is perpendicular to the rotational axis O.
The laser beams LK to LC deflected on the mirror face of the polygon mirror 101 get closer to the plane Q as they approach the position P, and intersect with one another at the position P after passing through the toroidal lens 102. The laser beams LK to LC separate from one another with distance from the position P and pass through the f.theta. lens 103. By means of redirecting mirrors 104aK to 104cK, 104aY to 104cY, 104aM to 104cM, and 104C, light paths of the laser beams LK to LC are redirected so that light path lengths are made equal. Then, the laser beams LK to LC respectively exposes surfaces of photosensitive drums 201K to 201C to form electrostatic latent images, through dust-proof glasses 105K to 105C.
Accordingly, the polygon mirror 101 and the scanning lens including the toroidal lens 102 and the f.theta. lens 103 are shared by all laser beams. This contributes to cost reduction and to the simplification of the apparatus.
Using this conventional multibeam scanning device, however, the laser beams LK to LC intersect with one another at the position P located immediately before the f.theta. lens 103. As a result, the laser beams LK to LC separate from one another after passing through the f6 lens 103 in the sub-scanning direction, with only a short distance in between, thereby causing a difficulty to discriminate the laser beams LK to LC.
The laser beams LK to LC should be reliably separated from one another before reaching the photosensitive drums 201K to 201C. Otherwise, insufficient exposure may occur to at least one of the photosensitive drums 201K to 201C, or noises may be caused on a reproduced image. For efficient operation, the separation of the laser beams should be performed using the redirecting mirrors 104aK, 104aY, 104aM, and 104C. However, the laser beams are separated from one another, with a short distance in between as described above, so that there would be a problem if these mirrors are set near the position P.
To address this problem, the redirecting mirrors 104aK, 104aY, 104aM, and 104C can be set away from the position P. Yet, if doing so, a distance between the position P and these mirrors increases, making the whole distance between the mirrors and the polygon mirror 101 longer. In spite of the reduced number of the polygon mirror and the scanning lens, the apparatus is consequently increased is size.
Alternatively, a special lens, such as a toric lens, can be set at the position P for separating the laser beams LK to LC in the sub-scanning direction, so that each distance between the neighboring laser beams increases and the laser beams LK to LC are separated near the position P. In this case, although the distance between the position P and the redirecting mirrors can be shortened, the distance between the position P and the polygon mirror cannot be shortened. In addition, a price of the apparatus may be high.
This problem occurs not only to a multibeam scanning device provided in a tandem-type color copying machine having photosensitive drums, but also to that provided in an image forming apparatus having a construction for forming multicolor images using laser beams.